B6: Inheritance, Variation and Evolution

• DNA

  The chemical that all of the genetic material in a cell is made up from
• DNA

  • It contains coded information - basically all the instructions to put an organism together and make it work
  • It determines what inherited characteristics you have
• Chromosomes


  Long molecules of DNA found in the nucleus of animal and plant cells
• Chromosomes normally come in pairs
• DNA

  A polymer made up of two strands coiled together in the shape of a double helix
• Gene

  A small section of DNA found on a chromosome that codes for a particular sequence of amino acids to make a specific protein
• Only 20 amino acids are used, but they make up thousands of different proteins
• Genes tell cells what order to put the amino acids together
• DNA determines what proteins the cell produces, which in turn determines what type of cell it is
• Genome

  The entire set of genetic material in an organism
• Understanding the human genome

  • Allows scientists to identify genes linked to different diseases
  • Helps understand inherited diseases and develop treatments
  • Allows tracing the migration of human populations
• DNA molecules contain genes that code for proteins
• Nucleotides

  The building blocks that make up DNA strands
• DNA structure

  • Consists of sugar, phosphate groups, and one of four different bases (A, T, C, G)
  • The bases always pair up in a complementary way (A with T, C with G)
• Amino acids

  The building blocks of proteins, coded for by sequences of three DNA bases
• There are parts of DNA that don't code for proteins, some of which control whether genes are expressed
• Protein synthesis

  1. DNA in the nucleus

  2. mRNA carries code to ribosomes in cytoplasm

  3. Ribosomes assemble amino acids into proteins
• Types of proteins

  • Enzymes
  • Hormones
  • Structural proteins
• Mutation

  A random change in the genetic code
• Mutations


  • Can sometimes be inherited
  • Can be caused by exposure to certain substances
  • Can change the protein coded for by a gene, affecting its function
• Types of mutations

  • Insertions
  • Deletions
  • Substitutions
• Sexual reproduction

  Combines genetic information from two parents to produce genetically different offspring
• Sexual reproduction

  • Involves fusion of male and female gametes, each with half the normal number of chromosomes
  • Produces variation in the offspring by mixing parental genetic information
• Asexual reproduction


  Produces genetically identical offspring from a single parent
• Meiosis

  1. Produces gametes with half the normal number of chromosomes

  2. Involves duplication of chromosomes followed by two cell divisions
• Meiosis


  Process of mixing of chromosomes and genetic variation between parent and offspring
• The offspring are genetically identical to the parent - they're clones
• Bacteria, some plants and some animals

  • Reproduce asexually
• Meiosis reduces the number of chromosomes by half
• Meiosis

  1. Chromosome duplication

  2. Chromosomes line up in pairs

  3. Chromosomes pulled apart

  4. Four gametes produced, each with half the normal number of chromosomes
• Meiosis only occurs in sex cells (gametes) in humans
• The cell produced by gamete fusion replicates itself by mitosis to produce an embryo
• As the embryo develops, the cells start to differentiate into specialised cell types
• Sexual reproduction

  Produces offspring with genetic variation from both parents
• Asexual reproduction


  Produces genetically identical offspring
• Asexual reproduction methods

  • Runners in strawberry plants
  • Bulbs in daffodils
• X and Y chromosomes

  Determine whether an organism is male (XY) or female (XX)
• Gamete formation

  1. X and Y chromosomes separate during meiosis

  2. Sperm have 50% chance of X or Y

  3. Eggs all have X chromosome
• Genetic diagrams show the possible gamete combinations and offspring genotypes
• Dominant and recessive alleles


  • Only one allele needs to be present for dominant traits to be expressed
  • Both recessive alleles need to be present for recessive traits to be expressed